Microelectromechanical systems (MEMS) are small devices made of electrical and mechanical components, designed to work together to sense physical properties in their local environment. For instance, MEMS pressure sensors are designed to sense and report the pressure of a fluid or environment in which the pressure sensor resides. MEMS pressure sensors can be capacitive pressure sensors.
Capacitive pressure sensors are made of two electrodes separated by a gap. When pressure is applied to a pressure diaphragm, the diaphragm deforms and the gap changes, allowing the pressure sensor to detect the pressure change. The two electrodes must be electrically separated for the capacitive pressure sensor to work.
Traditional capacitive pressure sensors use two wafers made of different materials for the electrode assembly, typically silicon for the diaphragm electrode wafer, and an insulator, such as glass, for the second electrode wafer. The glass electrode wafer is metalized to provide a conductive path for the second electrode. However, these types of capacitive pressure sensors result in thermal mismatch between the glass electrode wafer and the silicon electrode wafer it is attached to. Moreover, glass tends to move or creep, creating thermal hysteresis effects in devices. These characteristics cause errors which reduce the accuracy of the pressure sensor.
Other capacitive pressure sensors use an all silicon second electrode by using a silicon on insulator (“SOT”) wafer. In this type of arrangement, the second electrode is silicon separated from the first electrode by a thin dielectric layer, typically an oxide. While this arrangement avoids some of the pitfalls of a glass electrode, it has large parasitic capacitance, resulting in signal dilution. This occurs due to both the high permittivity of oxides and large bond area between the wafers. Additionally, the structure of diaphragm based capacitive pressure sensors results in the maximum gap deflection on only the small central portion of the diaphragm. This small area between the first and second electrode is the only useful area for measuring pressure, the rest of the area contributes to the parasitic capacitance.
Ideally, a capacitive pressure sensor design should avoid thermal mismatch between materials and minimize parasitic capacitance.